A nasty dispute between the Democratic National Committee and Bernie Sanders’ campaign deepened on Friday, as the campaign lashed out at party leaders for blocking its access to voter data and then filed a lawsuit claiming it was "sustaining irreparable injury and financial losses."

The DNC move came in retaliation for the Sanders' staff allegedly spying on Hillary Clinton campaign files.

“The leadership of the Democratic National Committee is now actively attempting to undermine our campaign,” Sanders campaign manager Jeff Weaver said at a press conference Friday afternoon. “This is unacceptable.”

Later Friday, Sanders' campaign filed a lawsuit in federal court seeking to regain access to the DNC voter database.

The lawsuit claimed the campaign is now "sustaining irreparable injury and financial losses."

The Washington Post first reported late Thursday that the Sanders camp acknowledged a low-level staffer viewed Clinton campaign voter data and was fired as a result. The DNC, in response, blocked Sanders campaign access to the party's master list of likely Democratic voters until it provides a full explanation.

In doing so, the DNC stirred up long-simmering complaints among Clinton’s rivals that some in party leadership are privately boosting the Democratic front-runner.

“Individuals leaders of the DNC can support Hillary Clinton … but they are not going to sabotage our campaign,” Weaver said.

He accused party leadership of unfairly locking down information the campaign itself obtained. He said that if the DNC continues to “hold our data hostage,” they will bring the DNC to court.

The DNC, though, fired back.

Chairwoman Rep. Debbie Wasserman Schultz said the Sanders campaign “inappropriately and systematically accessed Clinton campaign data, and in doing so violated the agreement that all the presidential campaigns have signed with the DNC.”

She said that when the campaign provides a full accounting, “we will make a determination on re-enabling the campaign's access to the system.”

The DNC rents out the master list in question to national and state campaigns, which add their own information compiled by volunteers and field workers.

Being shut out of seeing the list for any length of time would be a major blow to Sanders, who is attempting to cut into Clinton's sizable lead in the race for the Democratic presidential nomination.

Sanders spokesman Michael Briggs placed the blame for the incident in large part on the software vendor, NGP VAN.

“Sadly, the vendor who runs the DNC's voter file program continues to make serious errors. On more than one occasion, the vendor has dropped the firewall between the data of different Democratic campaigns,” he said in a statement to Fox News. While saying it was “unacceptable” for a campaign staffer to access “some modeling data from another campaign,” he also said they want to work with the DNC and vendor to fix the “software flaws” that could make Sanders’ records vulnerable as well.

Another campaign official told the Post that the Clinton data was never downloaded or printed.

But the plot thickened as The Associated Press reported that, according to an unnamed source, a summary of computer logs showed four aides to Sanders' presidential campaign – not just one -- accessed voter data compiled by the Clinton campaign.

Meanwhile, Stu Trevelyan, the NGP VAN’s CEO, told the Post the breach was an "isolated incident that was fairly short in duration ... By lunchtime, it was resolved."

“We were informed that our proprietary data was breached by Sanders campaign staff in 25 searches by four different accounts and that this data was saved into the Sanders' campaign account," Hillary for America National Press Secretary Brian Fallon said in a statement Friday.

The campaign asked "that the Sanders campaign and the DNC work expeditiously to ensure that our data is not in the Sanders campaign's account and that the Sanders campaign only have access to their own data."

Fox News' Whitney Ksiazek and Mike Emanuel and the Associated Press contributed to this report.